1. Field of the Invention
This invention relates to color video display apparatus and in particular to such display apparatus employing light valves.
2. Description of Related Art
Video projection display apparatus are commonly used to produce images which are larger than can be economically produced by direct-view display apparatus, such as direct-view cathode-ray-tube and liquid-crystal display apparatus. One of the most popular projection display apparatus employs three small (e.g. 7-inch diameter) monochrome cathode-ray tubes, for producing respective images consisting of red, green and blue pixels of a composite color image. These three monochromatic images are combined by a system of lenses which also focuses the resultant full-color image onto a large (e.g. 52-inch diagonal) screen.
A proposed successor to the three-CRT color projection display apparatus is a single-panel color projection display apparatus, such as is disclosed in European Patent Application 0 492 721 A2, which was published on 1 Jul. 1992 and which corresponds to pending U.S. patent application Ser. No. 218,882 filed on 25 Mar. 1994, both of which are hereby incorporated by reference. This single-panel display apparatus eliminates a number of problems associated with multiple-CRT projection display apparatus, such as misregistration of multiple mono-chromatic images on a screen and the need for stable high-voltage power sources for the CRTs. The single-panel display apparatus also has the advantage that it can be used in either projection or direct-view applications.
A preferred embodiment of the single-panel color projection display apparatus is illustrated in FIG. 1. The apparatus comprises a source 10 of intense white light which is reflected by a mirror 12 through rectangular-beam-shaping means onto a dichroic mirror system 14. The dichroic mirror system splits the impinging rectangular beam of white light into separate rectangular component bands of red, green, and blue light and directs these three colored light bands onto a rotatable prism assembly 16. Upon rotation, the prism assembly repeatedly scans the red, green, and blue bands through relay lenses 18 and 20, which image the spatially-separated scanning colored light bands onto a light valve panel 22 having an array of pixels. The scanning colored light bands are separated from each other by one-third of the panel height. Each time a light band of one color leaves the bottom of the array a corresponding light band of the same color appears at the top of the array and begins its scan.
The light valve panel 22 comprises a variable-transmissivity matrix array of pixels arranged in rows and columns. As used herein, the word "transmissivity" includes both light transmission from a surface (i.e. reflection) and transmission through a medium typical panel of this type is the well known liquid-crystal display (LCD) panel. The light valve panel modulates the bands of light, in response to applied video data, and forms a color image which is focused by a projection lens 24 onto a screen (not shown).
Before each colored light band passes over a respective row of pixels, the video data for that row must be applied to the column conductors of the array and the row must be selected. Because three different rows will be illuminated substantially simultaneously by the three different colored light bands, either three separate column conductors and drivers must be provided for each column of pixels or the data must be provided sequentially to the column conductors at three times the video line rate. The former approach further complicates the row and column drive electronics topography of an already-complicated light valve 22. The latter approach avoids that complication, but has been found to have limitations, particularly in the realm of high-definition television (HDTV) where the potential image quality and the video line rate are higher than those of conventional television.
A first limitation evidences itself by an artifact which appears as ghosts in the displayed image. The visibility of these ghosts increases with contrast and with increased video line rates. As a simple example, if a displayed image comprises a screen full of text, an original line of text at the top of the screen may reappear one-third of the screen height from the top of the screen as a first-order ghost, and may again appear two-thirds of the screen height from the top of the screen as a second-order ghost. In general, an original line of video and the first and second-order ghosts of the video line will be sequentially displayed at positions which are separated by approximately one-third of the total number of displayed video lines. In moderate to high-contrast images, a ghost becomes objectionable if it has a light output which exceeds about 1% of the average light output of the overall image presented on the screen. Typical ghosts experienced in producing moderate to high-contrast images with the single-panel display apparatus have brightnesses (relative to the average instantaneous light output of an image presented on the screen) of about 2% (for first-order ghosts) and 0.02% (for second-order ghosts). Such first-order ghosts generally annoy viewers. The second-order ghosts are not generally annoying, but are visible.
Additional limitations relate to the color accuracy and the brightness accuracy of the image produced by the single-panel display. While not so inaccurate as to be annoying, or even noticeable to a casual viewer, there are substantial, measurable errors in both the color and the brightness of the image.